[API] Fixed Ruby 1.8 syntax incompatibility in Rakefile
Remove dependency on "elasticsearch-extensions" in Ruby 1.8

Since the gem is used mainly for launching the testing cluster,
remove the dependency on Ruby 1.8.

Also, remove the related statements and expressions throughout
the test suites.
Remove dependency on "jbuilder" in Ruby 1.8
Added small tweaks to the main Rakefile